Leptin expression is rhythmic in brain and liver of goldfish (Carassius auratus). Role of feeding time

• Leptin expression follows a daily rhythm in the hypothalamus and liver of fed fish.
• A postprandial increase of leptin expression occurs in the liver but not in the brain.
• Feeding time by itself is not the only cue that regulates leptin expression rhythms.
• Leptin rhythms are not directly driven by clock gene (per1a, per3 and cry3) rhythms.

Daily rhythms of feeding regulators are currently arousing research interest due to the relevance of the temporal harmony of endocrine regulators for growth and welfare in vertebrates. However, it is unknown the leptin circadian pattern in fish. The aim of this study is to investigate if leptin (gLep-aI and gLep-aII) expression is rhythmic in goldfish (Carassius auratus) liver and brain, and if such rhythms are driven by feeding time through a food entrainable oscillator. Fish maintained under 12-h light:12-h dark photoperiod and a scheduled feeding time showed 24-h locomotor activity and glycaemia rhythms. Moreover, hepatic gLep-aI and brain gLep-aI and gLep-aII expression were rhythmic with different daily profiles, showing a postprandial increase of leptin expression in the liver but not in the brain. Under constant light and different feeding regimes (scheduled fed at 10:00, 22:00 or randomly fed), feeding time synchronized daily rhythms in locomotor activity, glycaemia and clock gene expression (gPer1a, gPer3 and gCry3), but the rhythmic expression of hepatic gLep-aI and brain gLep-aII only remained in fed fish at 10:00. In summary, daily rhythms of leptin expression in goldfish are differently regulated at central and peripheral level, and they are not directly driven by clock genes. The role of food entrained oscillators on leptin expression rhythms in fish remains to be demonstrated.